on the same subnet (same LAN or WAP). You would need to establish an "ad-hoc network". Here are a few links

XP http://www.microsoft.com/windowsxp/using/networking/setup/adhoc.mspx

Vista http://technet.microsoft.com/en-us/magazine/dd296746.aspx

Windows 7 http://www.kombitz.com/2009/02/09/windows-7-how-to-create-an-ad-hoc-network/

Yes you would only need to install SeaClear on the main laptop.

Search for Panasonic Toughbook CF-08 -- that's exactly what you want but they don't make them anymore. I picked one up a few years ago for $500 when it was considered dead stock. I don't use it now but I'm not ready to part with it either. You might find one on eBay if you set up a saved search, but frankly I think your best bet today would be to get a used iPad from some early adopter who HAS to have the newly-released iPad version, even though his old one is just fine. Not sunlight readable but they make weather-proof sleeves for it.